This invention relates to on-board diagnostics and, more particularly, to an on-board diagnostic method and system for diesel vehicles that determines the functionality of a catalyst as well as secondary fuel injection operation.
It is necessary to add reductant to a lean NOx catalyst under highly oxidized environment of diesel exhaust to reduce NOx emissions. When the reductant (typically diesel fuel) is injected into the exhaust gas, the burning of hydrocarbons across the catalyst releases heat, which increases the post-catalyst exhaust gas temperature. This resulting temperature rise and the characteristics of the temperature rises are related to the amount of fuel injection, the catalyst activity, exhaust flow, and other thermal properties of the catalyst system. For a given vehicle, the thermal properties of the exhaust system is assumed fixed. At the beginning, the catalyst and the secondary fuel injection system are assumed to be fresh and fully functional. Under these conditions, the degree of temperature rise, and the catalyst light-off temperature of HC are close to the theoretical values. As the vehicle mileage increases, the catalyst activity may degrade with time and the reductant delivery system may fail for some reasons. In order to ensure compliance of the emission standards, two key elements must be monitored continuously as required by OBDII. Analysis of the catalyst reveals that as catalyst ages, the light-off temperatures of NOx, HC, and CO increase, and the NOx, conversion efficiencies decrease. However, the peak conversion efficiencies for both HC and CO remain approximately the same as shown in FIGS. 1 and 2. For these reasons, the catalytic exotherm may remain the same, but the catalyst temperature to produce that exotherm increases. Thus, to evaluate the catalyst activity, the light-off temperature must be determined, in particular, the HC light-off temperature. To determine the HC light-off temperature, a series of exotherm vs. pre-catalyst temperatures (T) needs to be measured during fuel injection and temperature ramp-up excursion. The HC light-off temperature (LT) is defined as the temperature where temperature rise is equal to 50% of the predicted exotherm. Due to the heat transfer process and thermal properties of the catalyst system, there exists a delay time (dt=time to reach 95% of the expected temperature change) for the post-catalyst temperature (PT) to reach certain temperature after fuel injection. For a given catalyst system, the delay time is most sensitive to the exhaust flow rate and the temperature difference between the pre- and post-catalyst (PTxe2x88x92T). This delay time, dt, can be determined experimentally for each vehicle to fine-tune a simplified thermal model established. See, for example, xe2x80x9cA Simplified Approach to Modeling Exhaust System Emissions: SIMTWCxe2x80x9d, P. M. Laing, M. D. Shane, S. Son, A. A. Adamczyk and P. Li, SAE, 1999-01-3476.
In accordance with the present invention, a diesel diagnostic method is proposed that detects the operation of secondary fuel injection and monitors catalyst activity by monitoring the magnitude of temperature rise and the HC light-off temperature of a catalyst system due to exotherm generated by the after treatment reductant injection.
More particularly, an on-board diagnostic method is proposed for detection of the functionality of the diesel emissions control system that permits a continuous operational check of the secondary fuel injection system and the catalyst activity. This is accomplished by monitoring the HC light-off temperature and the magnitude of temperature rise, MTR, due to the exotherm generated by the catalysts in an active lean NOx catalyst system. MTR generated by the catalysts is related to the amount of secondary fuel injection, exhaust flow rate, and catalytic conversion of HC. The light-off temperature, LT, is related to the activity of the catalyst. By continuously monitoring these two parameters with reference to theoretical values stored in an engine control computer on the vehicle, the operation of the secondary fuel injection system and catalyst activity can be evaluated. When either of the parameters falls below a predetermined threshold value, the vehicle emissions may exceed emission standards and, if so, the system causes a malfunction indicator light (MIL) to be energized to advise the driver of the need for service.